PRBS generator selection in modem communication

ABSTRACT

The present invention relates to a system and method for selecting one of plurality of PRBS generators for use with a modem. The modem includes a measuring device adapted to measure an operating environment of the modem; and a storage device adapted to store a list of PRBS generator definitions. The modem selects one of a plurality of PRBS generators based on the measurement of the operating environment.

CROSS REFERENCE TO RELATED APPLICATIONS

[Not Applicable]

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[Not Applicable]

BACKGROUND OF THE INVENTION

One embodiment of the present invention relates to modems and morespecifically to MEDLEY symbols in ADSL modems.

Pseudo-random bit sequence (hereinafter referred to as “PRBS”)generators or scramblers have various uses in ADSL modems that conformto ITU-T Recommendations G.992.1 and G.992.2, both of which are herebyincorporated herein by reference. It should be appreciated that the termPRBS generator as used herein to identify either a pure PRBS generatoror a scrambler based on a PRBS generator. Specifically, two differentPRBS generators define the MEDLEY signal used for upstream anddownstream training. A different PRBS sequence defines the form of theREVERB signal used in training while user data are scrambled duringSHOWTIME using still another different PRBS generator.

Such a plurality of uses of PRBS generators in ADSL trainingdemonstrates a need for means in an ADSL modem to select one of a set ofdifferent PRBS generators appropriate to observations made duringtraining or to parameters that may not be known in advance.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention relates to a system and methodfor selecting one of a plurality of PRBS generators for use with amodem.

Another embodiment of the present invention provides a modem adapted toselect one of a plurality of PRBS generators. The modem includes adevice adapted to measure an operating environment of the modem andmemory adapted to store a list of PRBS generator definitions.

Yet another embodiment of the present invention provides a communicationnode adapted to establish a connection with a remote communication node.The node includes circuitry adapted to determine an operatingenvironment of the communication node, and a management information nodeadapted to control the communication device according a list of storedPRBS generator definitions. In one embodiment, the list of PRBSgenerator definitions is stored on the management information node. Inanother embodiment, the circuitry includes a memory communicating withat least the management information node and adapted to store the listof PRBS generator definitions.

Yet another embodiment of the present invention comprises a method forselecting one of a plurality of PRBS generators. The method includesmeasuring an operating environment of a modem and selecting a storedPRBS generator from the plurality of generators based on the measuredenvironment.

Still another embodiment of the invention comprises a method ofcommunicating where a determination is made of the number of carriers tobe used by a communication node. The number of carriers determined isthen compared to a threshold. If the number of carriers determined isless than the threshold, then a first PRBS generator is selected. If thenumber of carriers determined is greater than the threshold, then asecond PRBS generator is selected. The first PRBS generator may belonger than the second PRBS generator, for example.

Other aspects, advantages and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings, whereinlike numerals refer to like parts.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a generic communication systemthat may be employed in connection with one embodiment of the presentinvention.

FIG. 2 illustrates a method of selecting a PRBS generator in accordancewith one embodiment of the present invention.

FIG. 3 illustrates a generic shift register based PRBS generator of thetype employed in the present invention.

FIG. 4 illustrates another embodiment of a shift register based PRBSgenerator in accordance with one embodiment of the present invention.

FIG. 5 illustrates yet another embodiment of a shift register based PRBSgenerator in accordance with one embodiment of the present invention.

FIG. 6 is a flow diagram of one embodiment of the method of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG.1 illustrates a block diagram of a generic communication system thatmay be employed in connection with one embodiment of the presentinvention. The system comprises a first communication node 101, a secondcommunication node 111, and a channel 109 that communicatively couplesthe nodes 101 and 111. In one embodiment, the communication nodes areADSL modems for example, although any other type of transceiver devicethat transmits or receives data over a channel is contemplated.

The first communication node 101 comprises a transmitter 105, a receiver103 and a processor 106. The processor 106 may comprise, for example, amicroprocessor. The first communication node 101 is communicativelycoupled to a user 100 (e.g., a computer) via communication link 110, andto the channel 109 via communication links 107 and 108.

Similarly, the second communication node 111 comprises a transmitter115, a receiver 114 and a processor 118. The processor 118, likeprocessor 106, may comprise, for example, a microprocessor. The secondcommunication node 111 is likewise communicatively coupled to a user 120(again a computer, for example) via communication link 121, and to thechannel 109 via communication links 112 and 113.

During operation, the user 100 can communicate information to the user120 using the first communication node 101, the channel 109 and thesecond communication node 111. Specifically, the user 100 communicatesthe information to the first communication node 101 via communicationlink 110. The information is transformed in the transmitter 105 to matchthe restrictions imposed by the channel 109.

The transmitter 105 then communicates the information to the channel 109via communication link 107. The receiver 114 of the second communicationnode 111 next receives, via communication link 113, the information fromthe channel 109 and transforms it into a form usable by the user 120.Finally, the information is communicated from the second communicationnode 111 to the user 120 via the communication link 121. Communicationof information from user 120 to user 100 may also be achieved in asimilar manner. In either case, the information transmitted/received mayalso be processed using the processors 106/118.

One embodiment of the present invention provides a PRBS MEDLEY generatorfor use in ADSL modems, i.e., where ADSL modems are used, for example,as communication nodes 101 and 111 of the generic communication system.In one embodiment the ADSL modem selects one of a set of different PRBSgenerators appropriate to observations made during training or toparameters that may not be known in advance. An infinite number of PRBSgenerators are available for use in ADSL training. Specifications forsome of these PRBS generators are provided in Table 1 below.

It should be appreciated that an important aspect of a PRBS generator isthe initial state of the shift register. By specifying one of 2^(N)−1possible non-zero initial states, one of 2^(N)−1 possible PRBS sequencescan be selected for a given PRBS generator.

Further, a PRBS generator closely relates to a corresponding scramblerwith its input clamped to one. Specifically, the output of such ascrambler when its initial state is not all ones corresponds exactly tothe output of the corresponding PRBS generator when its initial state isnot all zeros, except that zeros and ones in the PRBS generator outputsequence are replaced, respectively, by ones and zeros in the scrambleroutput sequence.

In one embodiment, the MEDLEY signal is used in ADSL modems to estimatethe channel unit-sample response and/or the signal-to-noise ratio (SNR)profile of the channel, so that the pair of modems may negotiate a setof operating parameters that will best utilize the capacity available onthe channel. In order to accomplish this objective, the MEDLEY signalshould be pseudo-random. That is, it appears to be random to thechannel, but because the transmitter and the remote receiver shareknowledge of the exact sequence used, the modems are able to remove theeffect of the randomness in executing their signal processingalgorithms.

In one embodiment, specific PRBS generators are employed in ADSL modemsthat conform to the incorporated ITU-T Recommendations G.992.1 andG.992.2. One such PRBS generator generates repeated bit patterns thatrepeat every 511 bits in the downstream (i.e. central office-to-remote,denoted D/S) direction; another generates repeated bit patterns thatrepeat every 63 bits in the upstream (i.e. remote-to-central office,denoted U/S) direction.

While the 511-bit period of the downstream PRBS generator issufficiently random for purposes of modem training, the 63-bit period ofthe upstream PRBS generator is too short. The result is that theestimates of the channel unit-sample response and the channel SNR areinsufficiently accurate to predict the performance of the upstreamdirection of data transmission during SHOWTIME. Consequently, the modemsmust either add extra SNR margin to compensate for this missing channelknowledge or select a higher data rate that, necessarily, has lower SNRmargin. It should be appreciated that either approach results indegraded performance when compared to the ideal situation.

More particularly, the SNR estimation phase of the G.992.1 and G.992.2modems is performed during the R-MEDLEY and C-MEDLEY sequences, for theU/S and D/S channels, respectively. In one embodiment, the transmittersends pseudo-random QPSK data patterns across the DMT tones, enablingthe receiver to estimate the per-bin noise power. The per-bin noisepower is generally made up of two main contributors, namely, a linenoise term, including receiver and transmitter front-end noise andvarious cross-talk components, for example, plus an Inter-Symbol andChannel Interference (hereinafter referred to as “ISCI”) term.

The ISCI term results from imperfect channel equalization, which causesthe end-to-end channel impulse response (hereinafter referred to as“CIR”) to be longer than the length of the Cyclic Prefix (hereinafterreferred to as “CP”). The tails of the CIR outside the CP causeinterference from the previous and next DMT symbols (ISI=Inter-SymbolInterference) and among DMT tones (ICI=Inter-Channel Interference).

It should be appreciated that the ISCI term is by nature a deterministicinterference. For a given DMT symbol and tone, the ISCI term'sinstantaneous power depends on the particular QAM symbol sets that havebeen transmitted across the tones in current, previous and next DMTsymbols. In one embodiment, it has been found that an ideal estimate ofthe average power of the ISCI term requires averaging the instantaneouspower over all the possible combinations of the interfering QAM symbols.For example, if the ICI term dominates over the ISI, and 20 U/S bins areactive, during R-MEDLEY one would need 4¹⁹≈2.7×10¹¹ different DMTsymbols. In practice, if a maximum error of 0.5 dB is acceptable, muchshorter sequences are enough.

However, the sequence used for R-MEDLEY of the G.992.1 and G.992.2modems uses only 63 different QPSK symbols (it is periodic, with aperiod of 63 DMT symbols). It has been found that this is insufficientand indeed may result in large estimation errors, up to 2˜3 dB, forchannels dominated by ISCI. If for MEDLEY the SNR is overestimatedduring SHOWTIME, it has been found that the bit error rate would degradebelow the target value for a given margin. In particular, if the targetmargin is set below 2˜3 dB, it has been found that the bit error rate inSHOWTIME would be higher than 10⁻⁷, an unacceptable situation.

It should be appreciated that, for G.lite.bis and G.dmt.bis modems, aPRBS generator having a period much longer than 63 DMT symbols for thesequence (i.e., around 500 symbols) should be used during the U/S SNRestimation phase initialization.

One embodiment of the present invention includes selecting one of afamily or a plurality of PRBS generators for use in ADSL training isillustrated in FIG. 2. ADSL modems operate by making extensivemeasurements on their operating environment during an elaborate trainingprocedure that is executed when a pair of modems initially establishes aconnection, as indicated by FIG. 2. According to current standards, thePRBS generator employed at each stage of training is defined a priori.

According to one embodiment of the present invention, a list of PRBSgenerator definitions is stored in the memory of the modem or in themanagement information base (MIB) that control modem operation. Then,depending upon the results of measurements or observations made duringtraining (block 201), the modem (or MIB) selects the generator mostappropriate to the situation determined by the measurements orobservations (as illustrated by block 203).

Once a specific PRBS is chosen, the modem informs the remote modem ofits choice as indicated by block 205. One method for accomplishing thisstep is similar to those outlined in the commonly assigned in U.S.patent application Ser. No.09/882,100 titled “Method of IntelligentlyRestricting Symbol Size in ADSL Modems,” which application is herebyincorporated herein by reference.

A family of PRBS generators is specified that solves the problemoutlined above. As discussed above, any PRBS generator with a periodgreater than about 500 achieves the desired performance. The followingTable 1 provides a partial list of PRBS generators that meet thecriteria. TABLE 1 PRBS Generators of length 9, 10, . . . , 23 Basic PRBSInverse PRBS 9 4 9 5 10 3 10 7 11 2 11 9 12 6 4 1 12 11 8 6 13 4 3 1 1312 10 9 14 5 3 1 14 13 11 9 15 1 15 14 16 5 3 2 16 14 13 11 17 3 17 1418 5 2 1 18 17 16 13 19 5 2 1 19 18 17 14 20 3 20 17 21 2 21 19 22 1 2221 23 5 23 18

The rows in Table 1 provide a list of coefficients for shift registerbased PRBS generators, generally designated 300, similar to thatillustrated in FIG. 3 for example. The set of coefficients defining sucha PRBS generator has a general form as follows:

-   -   {c_(N),c_(N-1), . . . ,c₁},        and each row of Table 1 contains an entry i if c_(i)=1, and no        entry if c_(i)=0 for the generator corresponding to that row.        Such a PRBS generator of length N produces an output sequence        that repeats with a period of 2^(N)-1, so the repetition period        exceeds 500 whenever N exceeds 9.

FIG. 4 illustrates another embodiment of a PRBS generator, generallydesignated 400. In this embodiment, for example, the first row in Table1 corresponds to a set of coefficients as follows:

-   -   {c₉,c₈, . . . ,c₁}        with c₉=c₄=1 and c_(i)=0 for other values of i based on the        primitive polynomial1+x⁻⁴+x⁻⁹. FIG. 5 illustrates a shift        register based PRBS generator, generally designated 500, taken        from the last row of Table 1, the (23, 18) PRBS generator, based        on the primitive polynomial, 1+x⁻¹⁸+x³¹ ²³.

It should be appreciated that each PRBS generator has an inverse thatis, itself, a PRBS generator; the inverse PRBS generators. occupy theright side of Table 1.

FIG. 6 is a flow diagram of one embodiment of the method of the presentinvention, in which a PRBS generator is selected based on the number ofcarriers used in communication. Specifically, the system makes, duringtraining, a determination of the number of carriers to be used incommunication (block 601). The determination may be made by one of theprocessors 106 and 118 of FIG. 1, for example. Once the number ofcarriers is determined, the number is compared to a threshold (block603). If it is less than the threshold, a first PRBS generator isselected for use (block 605). If instead it is more than the threshold,a second PRBS generator is selected for use (block 607). In either case,the remote modem is informed of the selection (block 609).

Referring to FIG. 6, in general, the second PRBS generator is longerthan the first PRBS generator. The first PRBS generator may be, forexample, that shown in FIG. 4, which has a 511-bit period. The secondPRBS generator may be, for example, that shown in FIG. 5, which has aperiod of approximately 8 million bits. The threshold used may be 32,for example. Of course any number of different PRBS generators andthresholds may be used and are contemplated by the present invention. Inaddition, multiple thresholding may be used, so that more than two PRBSgenerators may be available for selection given channel conditions.

Many modifications and variations of the present invention are possiblein light of the above teachings. Thus, it is to be understood that,within the scope of the appended claims, the invention may be practicedotherwise than as described hereinabove.

1-21. (canceled)
 22. A communication device comprising: circuitryadapted to determine a characteristic of the communication device, andto select one of a plurality of PRBS generator definitions based on thecharacteristic; and wherein an indication of the selected PRBS generatordefinition is transmitted to a remote communication device.
 23. Thecommunication device according to claim 22 further comprising memoryadapted to store the plurality of PRBS generator definitions.
 24. Thecommunication device according to claim 23, wherein the circuitryincludes the memory.
 25. The communication device according to claim 22,wherein the circuitry comprises measuring circuitry.
 26. Thecommunication device according to claim 25, wherein the measuringcircuitry comprises a processor.
 27. The communication device accordingto claim 22, wherein the characteristic of the communication devicecomprises an operating environment of the communication device.
 28. Thecommunication device according to claim 27, wherein the operatingenvironment of the communication device comprises at least one channelcondition.
 29. The communication device according to claim 27, whereinthe operating environment of the communication device comprises apreselected criterion.
 30. A communication system comprising: a firstnode having circuitry adapted to determine a characteristic of the firstnode; a second node adapted to control the first node based on thecharacteristic of the first node and a plurality of stored PRBSgenerator definitions; and wherein an indication of a selected PRBSgenerator definition is transmitted to a remote communication device.31. The system according to claim 30, wherein the circuitry comprisesmemory for storing the plurality of PRBS generator definitions.
 32. Thesystem according to claim 30, wherein the circuitry comprises measuringcircuitry.
 33. The system according to claim 32, wherein the measuringcircuitry comprises a processor.
 34. The system according to claim 30,wherein the characteristic of the first node comprises an operationenvironment of the first node.
 35. The system according to claim 34,wherein the operating environment of the first node comprises at leastone channel condition.
 36. The system according to claim 34, wherein theoperating environment of the first node comprises a preselectedcriterion.
 37. The system according to claim 36 wherein the preselectedcriterion comprises a user selection.
 38. The system according to claim30, wherein the plurality of PRBS generator definitions are stored inthe second node.
 39. The system according to claim 30, furthercomprising memory communicating with at least the second node.
 40. Thesystem according to claim 39, wherein the plurality of PRBS generatordefinitions are stored in the memory.
 41. The system according to claim30, wherein the second node comprises a MIB.
 42. A method ofcommunication comprising: determining a characteristic of acommunication node; selecting, based on the determined characteristic, aPRBS generator from a plurality of stored PRBS generators; andtransmitting an indication of the selected PRBS generator to a remotecommunication device.
 43. The method according to claim 42, whereindetermining a characteristic comprises measuring at least one conditionof the communication node.
 44. The method according to claim 41, whereindetermining a characteristic comprises obtaining a preselectedcriterion.
 45. A method of communication comprising: determining acharacteristic of a communication node; comparing the determinedcharacteristic to a threshold; selecting a first PRBS generator if thedetermined characteristic is less than the threshold, and a second PRBSgenerator if the determined characteristic is greater than thethreshold; and transmitting an indication of the selected PRBS generatorto a remote device.
 46. The method according to claim 45 wherein thecharacteristic of the communication node comprises a number of carriersto be used by the communication node.